Compiling Blender - Python 3.9
make fails because of Python version, but I did not realise that Blender comes bundled with it's own Python package. So in the latest make update the code / scripts came bundled with latest Python version that they should be built against.
The following article gave me a hint for the solution.
Building Blender with a different Python version
I edited ./build_linux/CMakeCache.txt
changing  the all of the Python version number references
to that of the new version found in
./lib/linux_centos7_x86_64/python/bin/

I could then successfully built Blender from source again.